The invention relates generally to the field of directive gain antennas whereby the strength of the radiated electromagnetic field is concentrated in certain specified directions. In particular, the invention relates to vertically aligned gain antennas which are used in conjunction with a substantially horizontal ground plane and which use phasing apparatus to produce a predetermined current distribution along the antenna. Such gain antennas are generally well known and are end fed (excited) by a transmitter which supplies an RF signal to be radiated. Of course in receiver applications, the antenna end feed point is connected to a receiver and the antenna receives electromagnetic signals from the radiation pattern present in the transmitting medium.
The vertical antenna produces gain in a specified direction by arranging the current distribution along the radiating surface of the antenna such that inphase currents along the antenna will add and reinforce the radiation pattern in a given direction. The effect of out of phase currents is minimized by reducing their effective path length along the antenna. The out of phase current paths are shortened in length by phasing apparatus which effectively couple an inductance in series with the antenna at predetermined locations along its length. The gain of the antenna is related to the combined path length of all the inphase currents along the antenna.
In standard end fed antennas, the number of inphase current sections can be increased to produce more antenna gain by increasing the physical length of the antenna. However, the bandwidth of such a lengthened antenna is decreased because bandwidth varies inversely with the actual physical length that the radiating current effectively travels along the antenna, as measured in terms of electrical wavelengths. Thus in prior vertical gain antennas, the actual physical length of the antenna limits not only the actual antenna gain obtainable but also the bandwidth. For an antenna which requires a wide bandwidth, only a small amount of gain is practically realizable by prior gain antennas. The end feeding of gain antennas also results in input impedances which require complex broadband matching networks to provide a suitable impedance match between the antenna and the transmitter.
In one type of gain antenna phasing apparatus, a single metallic cylinder shorted at one end is used to produce an effective inductance at a point along a conductor, but such an inductance has an inherently narrow bandwidth. In another type of phasing apparatus, a reactance coil is placed in series with the radiating element, but this weakens the resultant antenna structure. In still another type, effective inductances are produced by reducing the diameter of the radiating surface so as to form pairs of sleeves. This structure limits the amount of inductance obtainable, results in a weak mechanical structure, and is difficult to initially manufacture and adapt for different placement along the radiating surfaces of similar antennas having different operating frequencies or different current distributions.